"To fight terrorism, you need people who know their language and culture. How are they going to be found if the entire Arab world opposes the war?"

This is an obscene statement.
The entire Arab world does not oppose "the war" - which may start if Saddam does not comply with UN resolutions. It's not even true that all Arabs are so afraid of death threats that they won't actually voice their opinion. The Arab world is not a monolith swayed by a small group of murderous thugs.
Al Qaeda's power lies in it's attack against the status quo. You could probably find a lot of Arabs who are dissatified with the status quo therefore Al Qaeda can be appealing to this large group of people.
US policy in regards to Iraq is also about changing the status quo and is much more likely to lead a better life for Arabs in the region than "the best case scenario" offered by Al Qaeda or the French govt.